The present invention relates to a facsimile machine adapted to be connected to a data transmission line which it shares with a terminal, comprising current limiting means adapted to be connected, at the input to the line and to transmit, to a data processing interface of the facsimile machine, differential signaling signals of the line in order to connect the interface directly with the line as a function of these signals.
A facsimile machine frequently shares with some other data reception terminal apparatus the use of a line of a telephone network to which it is connected. Such other apparatus may be a telephone answering machine connected, in the same manner as the facsimile machine, in parallel on the line, at the user of these apparatus. The cost of the connection to the telephone network is thus limited to the cost of a single line, but the telephone network no longer assures the switching of an incoming call to that one of the two apparatus which is concerned since there is only one telephone number.
Therefore, it is the calling apparatus which, after establishing a connection of by one of the machines, called "answering", must supply signaling signals which supplement the telephone number, such as, for instance an 1100 Hz frequency signal.
Thus, a calling facsimile machine gives off signaling signals in the form of an audible signal specific to a facsimile machine, which audible signal is detected by a listening or detection device of the facsimile machine called. The latter pre-empts by switching the incoming call to itself, connecting its modem (modulator-demodulator) to the line and, at the same time, actuating a switch in order to disconnect the answering machine from the line. In the absence of this specific audible signal, the answering machine, which answered the call, retains access to the line and handles the call, which is therefore a voice call.
However, the line is adapted to provide the signaling signals only to a single apparatus called. The other apparatus, in this case the facsimile machine, must therefore take up only a small part of the power present on the line in order not to weaken it and disturb the detection of it.
Therefore, for the phase of listening for the possible specific audible signal, prior to switching only the answering machine can "connect" in order to answer, and thus cause the sending of the specific audible signal, the facsimile machine being authorized to connect itself to the line only via its high-impedance listening device and not directly by its modem, which is low impedance.
It order to form the listening device, it is known to connect a transformer with high input impedance in parallel on the line. By differential effect, the transformer assures the rejection of the line voltages in common mode, that is to say, blocks the passage through its circuits of the voltages present equally on the two line wires, while it transmits the differential signaling signals, that is to say, signals which are measurable between the two line wires, to the modem, which is connected to its secondary.
This solution, however, is not satisfactory since the transformer has reduced impedance for the relatively low frequencies of the signaling signals. Although a transformer with windings having a large number of turns, which are bulky and costly, retains an impedance which is less reduced at low frequency, the impedance is however still sufficiently great to limit as desired the alternating current power taken from the line. It is therefore necessary to provide a switch in order, in the event of a call to the answering machine, to isolate the transformer from the line, so that it does not weaken the line signals when it is not active.
The present invention is directed at overcoming this drawback.